The present invention relates to a radiation tomography apparatus. More specifically, the present invention relates to a radiation tomography apparatus for generating multislice tomographic images.
There is known an X-ray CT (Computed Tomography) apparatus as a radiation tomography apparatus to generate sectional images of an imaging object using radiations such as X-ray. The X-ray CT apparatus is applied to human bodies and solid bodies as imaging objects and is used for a wide range of purposes including medical and industrial purposes.
The X-ray CT apparatus scans around an imaging object using its body axis direction as an axis and irradiates X-ray to the imaging object from an X-ray tube. A collimator shields to shape the X-ray irradiated from the X-ray tube and adjusts an irradiation range of the X-ray so that the X-ray is irradiated to an imaging area of the imaging object. An X-ray detection element of an X-ray detection array detects the X-ray penetrating the imaging object via the collimator. Based on detection data according to the detected X-ray, the X-ray CT apparatus generates a tomographic image for the imaging area of the imaging object.
The X-ray CT apparatus is subject to diversification with respect to locations of imaging objects and purposes of imaging. The X-ray CT apparatus needs to improve image quality such as resolutions and speed up imaging. To satisfy these demands, the X-ray CT apparatus is provided with an X-ray detection array. The X-ray detection array comprises a plurality of X-ray detection elements for X-ray detection that are disposed in an array form along a channel direction and a body axis direction. Available X-ray scanning systems include helical scanning and axial scanning. The axial scanning system irradiates X-ray around an imaging object for each section of an imaging area for the imaging object. The helical scanning system irradiates X-ray around the imaging object helically along the body axis direction.
FIG. 11 illustrates how an X-ray CT apparatus performs a scan according to the helical scanning system. The X-ray CT apparatus is provided with an X-ray detection array 123 comprising a plurality of X-ray detection elements 123a in an array form along the channel direction x and the body axis direction z. In FIG. 11, a plurality of X-ray detection elements 123a is disposed along the channel direction. The X-ray detection elements 123a comprise eight columns from A to H along the body axis direction z. In FIG. 11, FIG. 11(A) shows the start of scanning. FIG 11(B) shows scanning in process. FIG. 11(C) shows the end of scanning. As shown in FIG. 11, the helical scanning system irradiates X-ray around an imaging object helically along the body axis direction z.
As shown in FIG. 11 (A), the scan starts by aligning the irradiation center of an X-ray tube 120 and the center of the X-ray detection array 123 along the body axis direction with one end S of an imaging area R on an imaging object. At this time, the collimator 122 shapes an X-ray 105 from the X-ray tube 120 so that the X-ray forms a pyramid which is symmetrical about the irradiation center and has a specified thickness along the body axis direction z. The X-ray detection array 123 detects the X-ray 105 penetrating the imaging object via the collimator 122 using the X-ray detection elements 123a from columns E to H corresponding to the imaging area R of the imaging object.
During scanning as shown in FIG. 11(B), the X-ray tube 120 helically scans around the imaging object. For example, the X-ray tube 120 irradiates the pyramidal X-ray 105 symmetrical about the irradiation center from a direction opposite to the irradiation direction at the start of scanning. All columns from A to H of the X-ray detection elements 123a detect the X-ray 105 penetrating the imaging object via the collimator 122.
At the end of scanning as shown in FIG. 11 (C), the X-ray tube 120 helically scans around the imaging object. The X-ray tube 120 irradiates the pyramidal X-ray 105 symmetrical about the irradiation center from a direction similar to the irradiation direction at the start of scanning. The X-ray detection array 123 detects the X-ray 105 penetrating the imaging object via the collimator 122, using the X-ray detection elements 123a from columns A to D corresponding to the imaging area R on the imaging object. The scan ends by aligning the center of the X-ray detection array 123 along the body axis direction and the irradiation center of an X-ray tube 120 with the other end E of the imaging area R on the imaging object.
As mentioned above, an X-ray 105R is detected to generate a tomographic image at the start and the end of scanning. This is because the X-ray 105R corresponds to an area belonging to the imaging area R on the imaging object. However, X-rays 105S and 105E are not detected because they correspond to areas SS and SE outside the imaging area R and are not used for tomographic image generation. That is, at the start and the end of scanning, the X-rays 105S and 105E of the X-ray 105 irradiated from the X-ray tube 120 are incident outside the imaging area R from the center of the X-ray detection array 123. Consequently, the X-rays 105S and 105E are not used, and the X-ray 105 irradiated to the imaging object is not used efficiently.
Conventionally, various methods have been proposed to shield the X-rays 105S and 105E that belong to the X-ray 105 irradiated from the X-ray tube 102 and are incident opposite to the imaging area R of the center of the X-ray detection array 123 (e.g., see JP-A No. 234197/1997).
However, the conventional system uses the pyramidal X-ray 105 spreading along the body axis direction z in order to generate a tomographic image having a specified slice thickness. The system not only directly detects the X-ray through the imaging object from the X-ray tube 120, but also detects X-rays as scattered radiations resulting when the original X-ray penetrates the imaging object. This caused to decrease the tomographic image contrast and to generate an artifact. In particular, increasing a pyramid angle of the pyramidal X-ray causes many scattered radiations on the imaging object. For example, the imaging object's abdomen easily causes artifact due to ribs. Not only the helical scanning system, but also the axial scanning system has been subject to this problem.